Home' RTCA Documents for Review : DO-343B Contents © RTCA, 2018
In order to ensure that the drive level to the SSPAs is kept within the specified range,
and the linearity performance is maintained, a power control loop samples the current
driven by the L band SSPAs on the forward link, and controls the gain at the C band
receivers, whilst on the return link automatic level control circuits are used on the C
band SSPAs. The payload incorporates integrity checking units on both the forward
and return links which check the continuity of each mobile element path. For the forward
link, a C-band test carrier is fed to each L-band mobile element path, tapped off at the
ONET output, and routed to the C band Link Integrity Checker (CLIC). The CLIC senses
the signal at each of the 120 element paths, and feeds it to the C-band splitter, the
output of which is linked to the C-band downlink for transmission back to the SAS. This
allows the SAS to check the continuity of each transmit path with the minimum of on
board hardware. On the return link a similar process is used, but the approach taken
mimics that used on Inmarsat 3 satellites, whereby a test tone is generated by the
payload itself and transmitted to the SAS at C-band.
In addition to the main Communications Payload, the I4 satellites also carry a
Navigation Payload which is designed to support Satellite Based Augmentation System
(SBAS) operations. The payload transmits satellite navigation signals at the GPS L1
and L5 frequencies which allows the real-time relay from on-ground monitoring
networks of integrity and accuracy augmentation data for orbiting Global Navigation
Satellite Systems (GNSS) such as GPS and GLONASS.
Frequency Plan And Channelization
Like all previous generations of Inmarsat satellites, the I4 spacecraft use L band for
communication between the user terminals (UT) and the satellite, and C band for the
feeder link between satellite and the Satellite Access Stations (SAS). Table 3-2
presents the Inmarsat 4 frequency bands. Please also see Section 3.4.1.
1626.5 – 1660.5
6425.0 – 6575.0
1525.0 – 1559.0
3550.0 – 3700.0
Table 3-2: I4 Frequency Bands
The I4 satellite, through the DSP, provides up to 630 x 200 kHz bi-directional channels
for use in the C-to-L and L-to-C transponders. The DSP also allows for L-to-L links,
enabling direct UT-to-UT calls. In addition, the I4 satellites also include a separate
direct SAS-to-SAS link, providing 5 MHz of bandwidth, for direct C-to-C
On the feeder link the Inmarsat 4 satellites provide two global beams, one being RHCP
and the other being LHCP. The beams cover all the points within the satellite field of
view with elevation greater than 5 degrees.
The user link, using RHCP, provides a global beam, regional spot beams and narrow
spot beams. The global and regional spot beams are used to support some of the older
Inmarsat services, and also support call set-up procedures for the BGAN services. The
narrow spot beams are used by the new BGAN services. A 3-color frequency re-use
scheme is used for the regional beams, whilst a 4-color scheme is used on the more
densely packed narrow spots.
Figure 2-1 shows the coverage areas of the three I4 satellites. Figure 3-9 and Figure
3-10 show examples of regional beams and narrow spot beams respectively. Inmarsat
has the ability to reconfigure spot beams based on changing requirements.
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